<template>
    <transition :name="transitionName">
        <div class="back-to-ceiling" @click="backToTop" v-show="visible" :style="customStyle">
            <svg width="16" height="16" viewBox="0 0 17 17" xmlns="http://www.w3.org/2000/svg" class="Icon Icon--backToTopArrow" aria-hidden="true" style="height: 16px; width: 16px;">
                <title>回到顶部</title>
                <g>
                    <path d="M12.036 15.59c0 .55-.453.995-.997.995H5.032c-.55 0-.997-.445-.997-.996V8.584H1.03c-1.1 0-1.36-.633-.578-1.416L7.33.29c.39-.39 1.026-.385 1.412 0l6.878 6.88c.782.78.523 1.415-.58 1.415h-3.004v7.004z" fill-rule="evenodd"></path>
                </g>
            </svg>
        </div>
    </transition>
</template>

<script>
export default {
    name: "BackToTop",
    props: {
        scrollContainerClass: {
            type: String
        },
        visibilityHeight: {
            type: Number,
            default: 400
        },
        backPosition: {
            type: Number,
            default: 0
        },
        customStyle: {
            type: Object,
            default: function() {
                return {
                    right: "50px",
                    bottom: "30px",
                    width: "40px",
                    height: "40px",
                    "border-radius": "4px",
                    "line-height": "45px",
                    background: "#e7eaf1"
                };
            }
        },
        transitionName: {
            type: String,
            default: "fade"
        }
    },
    data() {
        return {
            visible: false,
            interval: null,
            wrapperElement: null
        };
    },
    mounted() {
        //获取监听的Div元素
        this.wrapperElement = document.getElementsByClassName(this.scrollContainerClass)[0];

        if (this.wrapperElement) { //监听Div Scroll 事件
            this.wrapperElement.addEventListener("scroll", this.handleScroll);
        } else { //监听Window Scroll 事件
            window.addEventListener("scroll", this.handleScroll);
        }
    },
    beforeDestroy() {
        if (this.wrapperElement) {
            this.wrapperElement.removeEventListener("scroll", this.handleScroll);
        } else {
            window.removeEventListener("scroll", this.handleScroll);
        }
        if (this.interval) {
            clearInterval(this.interval);
        }
    },
    methods: {
        handleScroll() {
            if (this.wrapperElement) {
                this.visible = this.wrapperElement.scrollTop > this.visibilityHeight;
            } else {
                this.visible = window.pageYOffset > this.visibilityHeight;
            }
        },
        backToTop() {
            const start = this.wrapperElement
                ? this.wrapperElement.scrollTop
                : window.pageYOffset;
            let i = 0;
            this.interval = setInterval(() => {
                const next = Math.floor(
                    this.easeInOutQuad(10 * i, start, -start, 300)
                );
                if (next <= this.backPosition) {
                    if (this.wrapperElement) {
                        this.wrapperElement.scrollTo(0, this.backPosition);
                    } else {
                        window.scrollTo(0, this.backPosition);
                    }
                    clearInterval(this.interval);
                } else {
                    if (this.wrapperElement) {
                        this.wrapperElement.scrollTo(0, next);
                    } else {
                        window.scrollTo(0, next);
                    }
                }
                i++;
            }, 16.7);
        },
        easeInOutQuad(t, b, c, d) {
            if ((t /= d / 2) < 1) return c / 2 * t * t + b;
            return -c / 2 * (--t * (t - 2) - 1) + b;
        }
    }
};
</script>

<style scoped>
.back-to-ceiling {
    z-index: 3000;
    position: fixed;
    display: inline-block;
    text-align: center;
    cursor: pointer;
}

.back-to-ceiling:hover {
    background: #d5dbe7;
}

.fade-enter-active,
.fade-leave-active {
    transition: opacity 0.5s;
}

.fade-enter,
.fade-leave-to {
    opacity: 0;
}

.back-to-ceiling .Icon {
    fill: #9aaabf;
    background: none;
}
</style>
